。
She turned pale and red by turns.
Literal
She [topic-は] blue became [and-たり] red became [and-たり] did.
~たり~たりする lists non-exhaustive examples of actions: here, 'turned pale and turned red (and so on).' The alternation between 青くなる (turning pale — from shock or fear) and 赤くなる (turning red — from embarrassment or anger) paints a picture of rapid emotional shifts visible on her face. This pattern is one of the most useful for describing multiple alternating or non-exhaustive actions.